The present invention relates to a method for preparing a building material. In addition, the invention relates to the building material, obtainable by said method. The invention further relates to a binder composition for preparing a building material.
Cement is a so-called hydraulic binder material that is widely used in the preparation of building materials. A particular popular and well known variety of cement is Portland cement. Portland cement is used in many applications such as mortar, concrete, and other building materials such as building blocks. Portland cement is produced by pulverizing clinker to a specific area of generally about 3000 to 5000 cm2/g. Clinker is created in a cement kiln at elevated temperatures from ingredients such as limestone, sand clay and fly ash. The cement kiln dehydrates and calcines the raw materials and produces a clinker composition comprised of tricalcium silicate (3CaO—SiO2), dicalcium silicate (2CaO—SiO2), tricalcium aluminate (3CaO—Al2O3) and tetracalcium aluminoferrite (4CaO—Al2O3—Fe2O3). The resulting clinker is typically ground to form fine dry cement powder. The finely ground cement generally is mixed with sand, coarse aggregate and water to produce mortars and concrete. Optionally, additives, such as plasticizers, may be added.
It is known to replace part of the Portland cement by slag material, such as granulated blast furnace slag. Blast furnace slag is a non-metallic product produced in the process of iron production. Blast furnace slag consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. Different forms of slag product are produced depending on the method used to cool the molten slag. These products include air-cooled blast furnace slag, expanded or foamed slag, pelletized slag and granulated blast furnace slag.
Granulated blast furnace slag is the glassy granular material, formed when molten iron blast furnace slag is rapidly chilled (quenched) by immersion in water. It is a granular product with very limited crystal formation. This type of slag consists primarily of silica (SiO2) and alumina (Al2O3) combined with calcium and magnesium oxides (CaO and MgO).
As mentioned above, ground granulated blast furnace slag is a cementitious material and has been used as a partial substitute for (Portland) cement. Post-processing blast-furnace slag to produce slag containing cement diverts it from the solid waste stream and creates a valuable product: it can substitute for a portion of Portland cement in concrete (usually from 20-80%, depending on the application), improving strength and durability. Utilization of slag cement not only lessens the burden on landfills, it also reduces air emissions at steel plants through the granulation process (as compared to the traditional air cooling process). In addition, by using alternative cementitious materials like slag cement to partially replace portland cement, the production of carbon dioxide, as well as the energy use is significantly reduced.
Ground granulated blast furnace slag is only “latently” hydraulic, i.e. it does not bind automatically after admixture with water. Thus, it requires the presence of an activator to initiate the hydration process. Currently, one of the most used activators is Portland cement. When using Portland cement, however, relatively large amounts of Ca(OH)2 are generated, which is an unstable and aggressive compound which causes many problems such as carbonation and resultant cracking of the concrete. Furthermore, concrete that has been produced with Portland cement will disintegrate at high temperatures (above 500° C.) as a consequence of the decomposition of the excess Ca(OH)2 to CaO and H2O, causing cracks and increased porosity.
Due to the important role of cement, concrete and concrete-like products in the engineering and construction industry, there is a continued need for improvements in the preparation and composition of cement and concrete. In particular, there is an ongoing search for cost-effective ways to prepare binder materials having improved binding properties for preparing stronger building materials.